Chapter 10.1Planetary Atmospheres

10.1 Atmospheric Basics

• Our goals for learning• What is an atmosphere?

• How do you obtain an atmosphere?

What is an atmosphere?

An atmosphere is a layer of gas that surrounds a world

How do you obtain an atmosphere?

– Gain volatiles by comet impacts

– outgassing during differentiation

– Ongoing outgassing by volcanoes

Keeping an Atmosphere

• Atmosphere is kept by the world’s gravity– Low mass (small) worlds= low gravity =almost no atm.– High mass(large) worlds = high gravity = thick atm.

• Gravity and pressure– Air pressure depends on how much gas is

there ie. The atmospheric thickness.

Gravity and Atmospheric Pressure• The stronger the gravity, the more gas is held by the world and the

greater the weight of atm. on a point

Earth’s Atmosphere• About 10 km

thick

• Consists mostly of molecular nitrogen (N2) and oxygen (O2)

Atmospheric Pressure

Gas pressure Gas pressure depends on both depends on both density and density and temperature.temperature.

Adding air Adding air molecules molecules increases the increases the pressure in a pressure in a balloon.balloon.

Heating the air Heating the air also increases also increases the pressure.the pressure.

What do atmospheric properties vary with altitude?

Light’s Effects on Atmosphere• Ionization: Removal

of an electron• Dissociation:

Destruction of a molecule

• Scattering: Change in photon’s direction

• Absorption: Photon’s energy is absorbed

Temperatures and composition changewith Height giving structure to an atmosphere

Earth’s Atmospheric Structure• Troposphere: lowest

layer of Earth’s atmosphere

• Temperature drops with altitude

• Warmed by infrared light from surface and convection

Earth’s Atmospheric Structure• Stratosphere: Layer

above the troposphere

• Temperature rises with altitude in lower part, drops with altitude in upper part

• Warmed by absorption of ultraviolet sunlight

Earth’s Atmospheric Structure• Thermosphere:

Layer at about 100 km altitude

• Temperature rises with altitude

• X rays and ultraviolet light from the Sun heat and ionize gases

Earth’s Atmospheric Structure• Exosphere: Highest

layer in which atmosphere gradually fades into space

• Temperature rises with altitude; atoms can escape into space

• Warmed by X rays and UV light

What have we learned?• What is an atmosphere?

– A layer of gas that surrounds a world

• How do you obtain an atmosphere?– comet impacts.– outgassing by differentiation, volcanoes,

• Why do atmospheric properties vary with altitude?– They depend on how atmospheric gases interact

with sunlight at different altitudes.

Atmospheric Processes 1

• Our goals for learning

• What are the key processes?

• How does a planet gain or lose atmospheric gases?

• How does the greenhouse effect warm a planet?

Atmospheric Processes

• Gaining and losing atmosphere– Gains: volcanic outgassing, impacts, evaporation.– Losses: gas escape,impacts,condensation,surface reactions

• Greenhouse Effect– Infrared energy is re-reflected back to the ground by CO2

• Atmospheric circulation (convection)– Convection cells move gas from equator to pole and back.

• Coriolis Effect– Gas dragged sideways by the rotation rate of the world.

Atmospheric Gains

Atmospheric Losses

Greenhouse Effect

Air Movement and Flow

What have we learned?• There are 3 ways of adding to atmosphere

and 4 ways of depleting it.– Gas molecules can transfer out to space or

down to the ground.

• How does the greenhouse effect warm a planet?– Atmospheric molecules allow visible sunlight to

warm a planet’s surface but absorb infrared photons, trapping the heat.

Atmospheric Processes 2 Weather and Climate

• Our goals for learning• What creates wind and weather?

Air MovementGas molecules move from high density to lower density

Atmospheric Pressure

Gas pressure depends on both density and temperature.

Adding air molecules increases the pressure in a balloon.

Heating the air also increases the pressure.(molecules more energetic)

Atmospheric Circulation (convection)

• Heated air rises at equator

• Cooler air descends at poles

Maximum Sun warming

Coriolis Effect

Coriolis EffectCoriolis effect deflects north-south

winds into east-west winds

Coriolis Effect breaks upGlobal Circulation

• On Earth the large circulation cell breaks up into 3 smaller ones, moving diagonally

• Other worlds have more or fewer circulation cells depending on their rotation rate

Coriolis Effect

Winds blow N or S Winds blow W or EWinds are diagonal

Venus EarthMars

Jupiter, Saturn Neptune, Uranus(?)

TotalAtmosphere Circulation

What have we learned?• What creates wind and weather?

– Atmospheric heating and Coriolis effect.

– Solar warming creates convection cells.– The coriolis effect drags winds sideways

and breaks up the cells– The faster a planet spins, the more E-W

gas movement there is

Weather and Climate• Weather is the ever-varying combination

of wind, clouds, temperature, and pressure– Local complexity of weather makes it difficult

to predict

• Climate is the long-term average of weather– Long-term stability of climate depends on

global conditions and is more predictable

Factors that can Cause Long-Term Climate Change

• Brightening of Sun

• Changes in axis tilt

• Changes in reflectivity

• Changes in greenhouse gases

Changes in Greenhouse Gases

• Increase in greenhouse gases leads to warming, while a decrease leads to cooling

Global Warming

Global Warming